Transmit diversity is one of the key technologies that define third generation wireless communication systems, such as cellular telephone networks. In such systems, spatial diversity is introduced into the signal by transmitting the signal through multiple antennas. Spatial diversity reduces the effects of channel fading by providing multiple independent copies of the signal at the receiver. With transmit diversity, the probability that all copies fade simultaneously in the channel is very small. As a result, the system performance is improved.
Transmit diversity can use multiple spatially separated antennas, as well as various temporal or frequency modulation techniques, or combinations of these techniques.
Space time transmit diversity (STTD) is an open loop transmit diversity technique. STTD has been adopted by the 3rd generation partnership project (3GPP) for wideband code division multiple access (W-CDMA) standards. Open loop means that there is no feedback about channel conditions from the receiver to the transmitter.
FIG. 1 shows a basic structure of a conventional STTD system 100. The system uses two transmit antennas 103, and one receive antenna 104. In such a system, the transmitter generates a stream of data to be transmitted as pairs of symbols X1 and X2 110. Each pair of symbols is fed to a symbol level STTD encoder block 101 of the transmitter. An output matrix 111 of the encoder 101 is
                                          C            1                    =                      [                                                                                X                    1                                                                                        X                    2                                                                                                                    X                    2                    *                                                                                        -                                          X                      1                      *                                                                                            ]                          ,                            (        1        )            where * denotes a complex conjugate. Transmit symbols, in a left-to-right order, of the rows, in a top-to-bottom order, are fed to the corresponding different transmit antennas, as shown in FIG. 1. That is, the transmit symbols X1, X2 of the first row all go to the first transmit antenna, and the transmit symbols X2*, −X1* of the second row all go to the second transmit antenna.
After passing through a wireless channel 115, the channel impulse responses h1 and h2 118 are detected and measured by the decoder 102 having a single antenna 104.
In general, this type of transformation maximizes the diversity gain in the case of two transmit antennas. Because the diversity gain is proportional to the number of transmit antennas, a higher number of transmit antennas are required to achieve higher diversity gains and to improve the performance of the system.
This is especially true in a downlink channel because a base station can typically incorporate a large number of transmit antennas. However, the 3GPP W-CDMA standards for the STTD scheme limits the transmitter to two transmit antennas. Therefore, any other transmit diversity technique for 3GPP, with a larger number of antennas, must achieve backward compatibility with systems with two transmit antennas.
Therefore, it is desired to provide STTD transmitters with more than two transmit antennas, while achieving backward compatibility with prior art two antenna 3GPP W-CDMA systems.